Report the real USB client connection state in companion builds

ArduinoSerialInterface::isConnected() always returned true, so any
companion build with a USB interface believes a client is attached from
boot onwards. Two user visible effects:

- new message notifications never fire, because MyMesh only notifies
  the UI (display, buzzer) while no client is connected
- on builds that also expose BLE the home screen shows '< Connected >'
  instead of the BLE pairing PIN, so a freshly flashed device cannot be
  paired at all

Add an optional connection check callback and wire it up per platform:
native USB-CDC (TinyUSB on nRF52/RP2040/ESP32 with USB_MODE=0) exposes
real DTR through (bool)Serial. The ESP32 USB-Serial-JTAG peripheral has
no DTR concept - it reports 'connected' as soon as the host enumerated
the device - so fall back to frame activity there. Plain UARTs keep the
previous assume-connected behaviour.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/helpers/ArduinoSerialInterface.h

@@ -4,23 +4,38 @@
 #include <Arduino.h>
 
 class ArduinoSerialInterface : public BaseSerialInterface {
+public:
+  // reports whether a client currently has this stream open (see setConnectedCheck)
+  typedef bool (*ConnectedCheck)();
+
+private:
   bool _isEnabled;
   uint8_t _state;
   uint16_t _frame_len;
   uint16_t rx_len;
+  uint32_t _last_frame_ms;
   Stream* _serial;
+  ConnectedCheck _conn_check;
   uint8_t rx_buf[MAX_FRAME_SIZE];
 
 public:
-  ArduinoSerialInterface() { _isEnabled = false; _state = 0; }
+  ArduinoSerialInterface() { _isEnabled = false; _state = 0; _last_frame_ms = 0; _conn_check = NULL; }
 
-  void begin(Stream& serial) { 
-    _serial = &serial; 
+  void begin(Stream& serial) {
+    _serial = &serial;
   #ifdef RAK_4631
     pinMode(WB_IO2, OUTPUT);
-  #endif  
+  #endif
   }
 
+  // Optional: let the target report the real link state, e.g. USB-CDC DTR.
+  // Without it isConnected() assumes true, as a plain UART has no way of knowing.
+  void setConnectedCheck(ConnectedCheck fn) { _conn_check = fn; }
+
+  // millis() of the last completely received frame, 0 if none since boot.
+  // Useful as an activity-based connection check where no DTR state exists.
+  uint32_t getLastFrameMillis() const { return _last_frame_ms; }
+
   // BaseSerialInterface methods
   void enable() override;
   void disable() override;